"Am I right that the rule is we just deliver a bunch of c,h,fx files and nothing else?" Yes, you are right.

"What's the difference between a 2D game and a 3D game ? I mean from a Lite-C point of view ?" A 2D game runs in a 2D space, not in a 3D space with a camera that looks down. You can picture a 2D game as a drawing on a piece of paper while a 3D environment would be (let's say) your room. On top of that, a 2D game can't use the psysics engine, the collision engine and so on.